Question: If $9a + 2b + c = 7$ and $3x + 7y = 9$, what is $54a - 49y - 21x + 12b + 6c$ ?
$= 54a + 12b + 6c - 21x - 49y$ $= (6) \cdot (9a + 2b + c) + (-7) \cdot (3x + 7y)$ $= (6) \cdot (7) + (-7) \cdot (9)$ $= 42 - 63$ $= -21$